This application is based on French Patent Application No. 00 10 190 filed Aug. 2, 2000, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to measuring atmospheric wind speeds using spatial wind lidar. The atmospheric wind speed is a fundamental component of operational meteorology; it is therefore of direct interest to scientists and to many commercial companies.
2. Description of the Prior Art
Obtaining terrestrial global measurements is no longer compatible with existing ground and airborne systems, which cover only a limited fraction of the terrestrial surface (oceanic areas in particular are very badly covered); it has therefore been proposed to carry out measurements from space, using Doppler lidar, to obtain sufficient meshing of measurements to satisfy the requirements stated by meteorologists, in particular of Eumetsat and Mxc3xa9txc3xa9o-France.
The principle of lidar (also known as optical radar) is analogous to that of radar: an optical wave emitted by a laser emitter is back-scattered by molecules and particles in the atmosphere to a receiver which is located, in the case of spatial lidar, near the laser emitter; the received signal is then fed to a detector and processor system and then transmitted to the ground for possible additional processing. The return signal is sampled in time to obtain information corresponding to each atmosphere layer through which it has passed: the lidar is therefore inherently a sounder. In the case of Doppler lidar, the processing of the signal entails using means that depend on the type of detection employed (direct, coherent, heterodyne, etc) to determine the frequency difference between the wavelength emitted by the laser and that of the return signal, which yields directly the projection onto the sight axis of the speed difference between the carrier of the emitter and the molecules and/or particles that back-scattered the laser signal; appropriate algorithms deduce from the speed difference information on the components of the speed of the wind in the sounded area: ideally, measurements along three different sight axes at a given point yield the three components of the wind vector at that point (two in the horizontal plane and the third along a vertical at the location concerned). The vertical speed of the wind can also be ignored, two measurements at each point being considered sufficient.
U.S. Pat. No. 5,367,371 describes the principle of atmospheric wind speed measurement by spatial lidar, as explained above. It suggests providing a satellite with two telescopes with sight angles of 45xc2x0 to the roll axis of the satellite. With regard to orbits, the document limits itself to describing the locations of the points of impact of the rays of the two telescopes for a given orbit of the satellite. The problem of terrestrial coverage is mentioned, but in point of fact merely in terms of it being a requirement, with no real solution being provided.
U.S. Pat. No. 5,872,621 concerns a solution to the problem of reception in spatial lidar; it proposes to use a holographic optical member to direct the back-scattered beam toward the receiver of the lidar. The holographic optical member is driven in rotation to scan the area to be covered. As in the previous document, this document refers to the coverage problem only to show the cycloid curve formed by the locus of the points of impact of the beam on the ground for a given trajectory of the satellite.
Additionally, various satellite observation methods based on the Doppler effect have been proposed. JP-A-10 19683 proposes, in a constellation of at least three satellites, sending laser pulses from one satellite to a mirror on other satellites rotating in opposite directions. The pulses are reflected toward the source satellite and the time difference between the pulses is measured; a measured value of the electromagnetic ether speed is deduced from this. U.S. Pat. No. 4,463,357 proposes to measure the electron content of the ionosphere between a spacecraft and a receiver by crossed correlation of two coherently modulated signals; using GPS satellites and a plurality of ground stations, it is possible to locate terrestrial events that give rise to ionospheric disturbancesxe2x80x94such as volcanic eruptions or the launch of intercontinental missiles. U.S. Pat. No. 5,428,358 proposes an analogous way to measure the electron content of the ionosphere using satellites of the GPS constellation. U.S. Pat. No. 5,675,081 proposes a system for measuring the water content of the atmosphere using water vapor radiometers and the satellites of the GPS constellation. The above documents are silent on the subject of measuring atmospheric wind speeds; nor do they mention the problem of determining orbits for such measurements.
Thus measuring atmospheric wind speeds gives rise to various problems. A first major problem lies in the weakness of the resources available on board a satellite (mainly in terms of electrical power), compared to those available on the ground, given the very severe demands of scientists both as to the accuracy of the wind speed measurement and the number of sight axes. A second problem is that of the geographical coverage of the measurements: a spatial system for measuring wind speeds using lidar mounted on a constellation of satellites would be of greater interest if it were able to provide:
the widest possible coverage of the terrestrial surface, and
the greatest number of measurements for each individual cell sounded, using different sight axes; a cell typically has the following dimensions: 200xc3x97200 km horizontally, with a vertical dimension of a few hundreds of meters, typically from 500 m to 1 km.
The invention proposes a solution to the various problems just mentioned. First of all, it uses a limited number of very simple measuring satellites. A preferred embodiment of the invention provides good terrestrial coverage. In a first configuration, the measurement points are distributed substantially regularly over half of the surface of the terrestrial globe, i.e. the sight axes of the instruments mounted on the various satellites are projected onto the ground at points which, combined with each other, constitute trajectories (referred to as tracks) whose intersections with the equator are quasi-equidistant. In a second configuration, over the other half of the surface of the terrestrial globe, the measurement points are grouped by areas (typically with dimensions less than 200xc3x97200 km), i.e. the sight axes of the instruments mounted on the various satellites are projected onto the ground at points contained within said areas. The two configurations are each produced over a period of 12 hours, at intervals of 24 hours, and the halves of the surface of the globe referred to vary in time at a rate that can be chosen by varying the altitude of the satellites. This embodiment of the invention therefore yields measurements that are easy for meteorologists to use.
The invention proposes a constellation of satellites for measuring atmospheric wind speeds, the constellation including at least two satellites distributed over the same non-geosynchronous orbit and each carrying a Doppler lidar.
The sight axis of the Doppler lidar of a satellite is preferably fixed. In one embodiment of the invention the orbit is a polar or quasi-polar orbit. It is also advantageous if the orbital altitude is from 350 km to 500 km.
In one embodiment the constellation includes three satellites and the orbital altitude is from 400 km to 500 km.
It is also advantageous if the sight angle of one satellite relative to the nadir is from 30xc2x0 to 50xc2x0 and is preferably about 45xc2x0.
In another embodiment the satellites of the constellation have different angles between the projection of the sight axis on the surface of the Earth and the projection of the speed of the satellite on the Earth.
In this case the constellation can include two satellites and the difference between the angles is then from 75xc2x0 to 105xc2x0 and is preferably about 90xc2x0.
The constellation can also include three satellites and in this case the difference between the angles is from 90xc2x0 to 150xc2x0 and is preferably about 120xc2x0.
The tracks on the surface of the Earth of the sight axes of the satellites of the constellation are advantageously substantially coincident in a first area of the surface of the Earth. The tracks on the surface of the Earth of the sight axes of the satellites of the constellation can also be substantially regularly distributed in a second area of the surface of the Earth.
In one embodiment the constellation satisfies the condition:                     2        ⁢        π        ⁢                  xe2x80x83                ⁢                  T          S                    nT        ⁢    R     less than                     H                  cos          ⁢                      xe2x80x83                    ⁢          γ                    ⁢              "LeftBracketingBar"                              tan            ⁢                          xe2x80x83                        ⁢                          α                              i                +                1                                      ⁢            sin            ⁢                          xe2x80x83                        ⁢                          ϕ                              i                +                1                                              -                      tan            ⁢                          xe2x80x83                        ⁢                          α              i                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          ϕ              i                                      "RightBracketingBar"              +                            T          S                T            ⁢      R      ⁢              xe2x80x83            ⁢              ψ                  i          ,                      i            +            1                                ≤      a    +                            2          ⁢                      xe2x80x83                    ⁢          π          ⁢                      xe2x80x83                    ⁢                      T            S                          nT            ⁢      R      
in which n is the number of satellites in the constellation, T is the period of rotation of the Earth, TS is the orbital period, R is the radius of the Earth, H is the orbital altitude, xcex3 is the angle between the projection of the speed of satellite t on the surface of the Earth on passing over the equator and the rotation speed of the earth at the equator, xcex1i is the angle between the sight axis of satellite i and the nadir, (Pi is the angle between the projection on the Earth of the sight axis of satellite i and the projection on the Earth of the speed of that satellite, "psgr"i,i+1 is the angle measured in the plane of the orbit between the satellites i and i+1, and a is a parameter analogous to a distance and is less than 500 km.
In this case it is advantageous if the parameter a has a value less than 100 km and preferably less than 50 km.
In another embodiment the angle between two satellites of the constellation in the plane of the orbit satisfies the condition:                     T        S            T        ⁢    R    ⁢          xe2x80x83        ⁢          ψ              i        ,                  i          +          1                      ≤            H              cos        ⁢                  xe2x80x83                ⁢        γ              ⁢          "LeftBracketingBar"                        tan          ⁢                      xe2x80x83                    ⁢                      α            i                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      ϕ            i                          -                  tan          ⁢                      xe2x80x83                    ⁢                      α                          i              +              1                                ⁢          sin          ⁢                      xe2x80x83                    ⁢                      ϕ                          i              +              1                                          "RightBracketingBar"        ≤                              T          S                T            ⁢      R      ⁢              xe2x80x83            ⁢              ψ                  i          ,                      i            +            1                                +    b  
in which T is the rotation period of the Earth, TS is the orbital period, H is the orbital altitude, xcex3 is the angle between the projection of the speed of satellite i on the surface of the Earth on crossing the equator and the rotation speed of the Earth at the equator, xcex1i is the angle between the sight axis of satellite i and the nadir, xcfx86i is the angle between the projection of the sight axis of satellite i on the Earth and the projection of the speed of the satellite on the Earth, "psgr"i,i+1 is the angle measured in the plane of the orbit between satellites i and i+1, and b is a parameter analogous to a distance and is less than 200 km.
In this case the parameter b has a value less than 20 km and preferably less than 10 km.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, which description is given by way of example and with reference to the accompanying drawings.